The invention relates to a display device having a display panel comprising at least one compartment which contains an ionizable gas mixture including at least a basic gas and an additional gas, the compartment being provided with electrodes for selectively ionizing the ionizable gas mixture during operation, and the display panel comprising means for supplying the additional gas to the ionizable gas mixture.
Display devices for displaying monochromatic images or color images comprise plasma-addressed liquid crystal display devices, referred to as PALC displays, preferably of the flat-panel type. PALC displays are used, for example, as displays for television and computer applications.
A display device of the type described in the opening paragraph is known from, for example published European patent application EP 0 816 898. The flat-panel type display device described in this application has a display screen with a pattern of (identical) data storage or display elements and a plurality of compartments. The compartments are filled with an ionizable gas mixture and are provided with electrodes for selectively ionizing the ionizable gas mixture during operation. In the known display device, the compartments have the shape of parallel, elongated channels in the form of a channel plate functioning as selection means for the display device. These are the plasma-addressed row electrodes. By applying a voltage difference across the electrodes in one of the channels of the channel plate, electrons are emitted from the cathode. The electrons will ionize the ionizable gas so that a plasma is formed. When the voltage across the electrodes in one channel is switched off and the gas mixture is deionized, a subsequent channel is switched on. At the display screen side of the display device, the compartments are closed by a relatively thin dielectric layer referred to as the microsheet. This layer is provided with electro-optical material, and further electrodes which function as the data electrodes or column electrodes of the display device. The further electrodes are provided on a substrate. The display panel is constituted by the assembly of the channel plate with the electrodes and the ionizable gas mixture, the dielectric layer, the layer of the electro-optical material and the further electrodes.
In a PALC display panel, the panel is addressed row by row. The resolution is determined by the number of rows which can be written per second. The minimally required time for writing a row is determined by the time required to ignite the plasma, the time required to charge the microsheet and the time required to switch off the plasma, referred to as the afterglow decay time. The shorter the afterglow decay time, the larger the number of rows which can be written per second and the higher the maximally achievable resolution.
A gas mixture which is known to be suitable for a relatively short afterglow decay time is a He-H.sub.2 mixture. However, a problem with such a mixture is the stability with respect to time. The quantity of H.sub.2 is relatively small (typically less than 3%) and the probability that the equilibrium pressure of H.sub.2 in the mixture remains constant is very small. This pressure variation is caused by the loss of hydrogen, for example, due to implantation in the electrodes during operation of the panel, or because there may be hydride formation. At a too low hydrogen pressure, the afterglow decay time will be too long, while the plasma will no longer function optimally at a too high hydrogen pressure. Said European patent application proposes providing the panel with a material which supplies and absorbs hydrogen so as to control the hydrogen pressure in the compartments.
A drawback of this solution is that, for the material used as a hydrogen source, the equilibrium pressure of hydrogen in the material is dependent on the temperature. Since the operating temperature of a PALC display panel may vary between 0.degree. C. and about 80.degree. C., a variation of more than a factor of 100 may occur in the hydrogen pressure, which is not acceptable.